Airbags for motor vehicles are known and have been used for a substantial period of time. These devices are installed on the driver and passenger side of automobiles and, in the event of a collision, are rapidly inflated with gas, to act as a barrier between the driver or passenger and the steering wheel or dashboard of the automobile.
Coatings have been applied to fabrics, intended for use in automotive airbags, to resist the unwanted permeation of air through the fabric and, to a lesser extent, to protect the fabric from detriment by the hot gases used to inflate the bags. Polychloroprene was the polymer of choice in the early development of this product, but the desire to decrease the folded size of the completed airbag, and the tendency of polychloroprene to degrade, with exposure to heat, and release the components of hydrochloric acid (thereby potentially degrading the fabric component as well as releasing hazardous chemicals), has led to the almost universal acceptance of silicone (polydimethylsiloxane or similar materials) as a more suitable coating. In the quest for the most compact folded size possible, coating levels of polymer have dropped from around 2.5 ounces per square yard of fabric, to levels approaching 0.5 ounces per square yard (for driver's side and passenger airbags that do not generally require long-term gas retention).
There are three primary types of different airbags, each for different end uses. Driver side airbags are generally mounted within steering columns and exhibit relatively low air retention in order to act more as a cushion for the driver upon impact. Passenger-side airbags also comprise relatively high air permeability fabrics that permit release of gas either by percolation of the gas through the fabric or through vents integrated therein. Both of these types of airbags (composed of multiple fabric panels) are designed to protect persons in sudden collisions and generally burst out of packing modules from either a steering column or dashboard. Side curtain airbags, however, have been designed primarily to protect passengers during rollover crashes by retaining the inflation state for a long duration and generally unroll from packing containers stored within the roofline along the side windows of an automobile. Side curtain airbags therefore not only provide cushioning effects but also provide protection from broken glass and other debris. Therefore, it is imperative that side curtain airbags, as noted above, retain large amounts of gas, as well as high gas pressures, to remain inflated throughout the longer time periods of the entire potential rollover situation. To accomplish this, these side curtains are generally coated with very large amounts of sealing materials on both the front and back faces. Since most side curtain airbag fabrics are comprised of woven blanks that are either sewn, or sealed, or integrally woven together, discrete areas of potentially high leakage of gas are prevalent, particularly at and around the seams. It has been accepted as a requirement that heavy coatings were necessary to provide the low permeability (and thus high leak-down time) necessary for side curtain airbags. Without such heavy coatings, such airbags would most likely deflate too quickly and thus would not function properly during a rollover collision. As will be well understood by one of ordinary skill in this art, such heavy coatings add great cost to the overall manufacture of the target side curtain airbags. There is thus a great need to manufacture low permeability side curtain airbags with less expensive (preferably lower coating add-on weight) coatings without losing the aging, humidity, and permeability characteristics necessary for proper functioning upon deployment. To date, there has been little accomplished, if anything at all, that alleviates the need for such thick and heavy air retentive coatings from side curtain airbags.
New developments in airbags, particularly newer designs being placed in the sides of the passenger compartment (i.e., side curtain airbags), have introduced the requirement that the bags hold pressure longer under use (for the purpose of protection during a potential rollover collision) than for driver's side and passenger airbags. Such rollover protection side curtain airbags (note: some side curtains are not designed for rollover protection and usually exhibit the same levels of gas permeability as the other standard types of airbags) generally require thicker coatings of the most trustworthy materials (e.g. silicone polymers).
Silicone polymers have excellent thermal properties but have relatively high permeability to gases, when compared to many other elastomers. This has not been a matter of concern in coatings used for driver side airbags, since the retention time requirements are very small, as noted above. The advent of side curtains, with higher air retention requirements is bringing to light the difficulties in achieving the desired retention properties at desired add-on levels.
The utilization of such silicone polymers has, in the past, come at a price. The costs associated with such silicone compounds are generally quite high, particularly the costs required to provide sufficient coverage of target fabrics while best ensuring low permeability will continue as long as necessary. Furthermore, although lower levels of other types of coatings (thermoplastics and thermosets, such as polyurethanes, for example) have been utilized for such a purpose, there are general add-on amounts that, to date, are required to provide needed long-term inflation gas retention rates for target silicone-coated airbag cushions. As stated above, silicone coating materials are generally preferred over other polymer types due to their ability to withstand varied environmental and storage conditions over long duration. Thus, it is highly desired to utilize a trustworthy, high inflation gas retention, coating for low permeability airbag cushions, particularly with relatively low costs involved in providing such benefits.
Furthermore, yarn shifting has proven to be a significant problem for such airbag cushions as well. When a sewn seam is put under stress, a naturally lubricating silicone coating may allow the yarns from which the fabric is constructed to shift. This shifting can lead to leakage of the inflating gas through the new pores formed from the shifting yarns, or, in drastic cases, cause the seam to fail. Since the airbag must retain its integrity during a collision event, in order to sufficiently protect the driver or passenger, there is a great need to provide coatings which provide both effective permeability characteristics and sufficient restriction of yarn shifting for the airbag to function properly, if and when necessary. Again, such a coating material is preferably silicone in nature for storage purposes. Therefore, a need exists to provide such beneficial characteristics at lower cost and/or lower add-on levels through an airbag coating that provides low permeability, resistance to yarn shifting and age resistance over long periods of storage.
As another issue, it has recently been found that more efficient side curtain airbag cushions may be produced as one-piece woven (preferably Jacquard woven) articles. Interestingly, the requirements for effective coatings for such one-piece woven airbags are significantly different from those needed for standard driver or passenger side airbags. A one-piece Jacquard (for example) airbag cushion is more economical to produce due to the elimination of the need to first cut fabric portions from coated webs and subsequently sew them together. The distinct disadvantage of this system is that the target bag must be coated on the outside during production, (as opposed to a sewn bag in which the coated face is normally placed within the interior of the air bag). When the Jacquard woven bag is then deployed, inflation pressures may be transmitted through the fabric to the coating, applying a potentially delaminating force to that coating and creating a potential problem. If the adhesion of the coating to the fabric is strong, then the diffusion forces are localized and, depending upon the strength of the coating film, may lead to a rupture of the film itself, whereupon the inflation gases can easily escape. If the airbag is intended as a side curtain, such inflation gas loss would severely reduce the effectiveness of the inflated airbag and jeopardize its ability to protect during a long duration rollover scenario. On the other hand, if the adhesion of the coating is less strong, then the diffusing force can be dissipated by localized delamination of the film without rupture thereof. This would typically result in a blister (known in the airbag coating industry as an aneurysm) wherein the inflating gases can be retained, but the appearance of the bag is objectionable, regardless of the fact that the bag itself most likely retains the inflation gases therein. Thus, coatings for such one-piece woven airbags must take into account this dichotomy and balance the adhesion of the coating with the retention of the inflating gases. To date, such a balance of considerations in developing proper airbag coatings, particularly for one-piece woven airbag cushions, has not been exercised.
Furthermore, there is a current drive to store such low permeability side curtain airbags within cylindrically shaped modules. Since these airbags are generally stored within the rooflines of automobiles, and the area available is quite limited, there is always a great need to restrict the packing volume of such restraint cushions to their absolute minimum. However, the previously practiced low permeability side curtain airbags have proven to be very cumbersome to store in such cylindrically shaped containers at the target automobile's roofline. The actual time and energy required to roll such heavily coated low permeability articles as well as the packing volume itself, has been very difficult to reduce. Furthermore, with such heavy coatings utilized, the problems of blocking (i.e., adhering together of the different coated portions of the cushion) are amplified when such articles are so closely packed together. The chances of delayed unrolling during inflation are raised when the potential for blocking is present. Thus, a very closely packed, low packing volume, low blocking side curtain low permeability airbag is highly desirable. Unfortunately, the prior art has again not accorded such an advancement to the airbag industry.